1. Field of the Invention
This invention relates to an AlGaAs semiconductor laser device which emits laser light from the facet thereof and it also relates to a method for the production of such a semiconductor laser device.
2. Description of the Prior Art
In recent years, AlGaAs and other types of semiconductor laser devices have widely come into practical use as light sources for optical disc driving units. When semiconductor laser devices are used as the light source for write-once optical disc driving units or rewritable optical disc driving units, they are required to have high reliability at a high output power level of from 40 to 50 mW. When used as the light source for optical pumping of solid-state laser devices such as YAG laser, an output power of 100 mW or more is required.
However, it has been reported that the reliability of the semiconductor laser devices in practical use today which can attain laser oscillation at a relatively high output power level is inversely proportional to the optical output power raised to the fourth power, when the devices of the same construction are compared. In other words, it is extremely difficult to raise the optical output power, while maintaining high reliability.
The principal cause for deterioration of semiconductor laser devices in high output power operation is facet deterioration. This is because heat is generated locally at the laser light emitting facet due to the high light density at this facet. The mechanism of this heat generation will be explained by reference to FIGS. 10a-10b and 11a-11b.
FIGS. 10a and 10b are schematic diagrams showing the energy band structures near the surface originating from the surface state which occur when either the n-type or p-type GaAs (110) surface is slightly oxidized. In the case of either n-type or p-type GaAs, numerous carriers accumulate near the surface to form a so-called "accumulation layer" which is indicated by reference numeral 1 in these figures.
In general, it is well know that the surface state will bend the energy bends near the surface. In addition to the accumulation layer 1 shown in FIGS. 10a and 10b, minority carriers may gather near the surface and majority carriers be distanced from the surface as shown in FIGS. 11a and 11b, resulting in the formation of an inversion layer 2 which is a local inversion of the conductivity type. Whether an accumulation layer 1, or an inversion layer 2 forms, is dependent on the height relationship between the surface state and the Fermi level of the semiconductor. In either case of n-type or p-type GaAs, an accumulation layer will form.
The electrons and positive holes trapped in the surface state E.sub.s are released after a short relaxation time, and this energy is released as heat. Electrons and positive holes are then again trapped in the surface state which has become a vacant state, and the above process is repeated, so that heat continues to be released.
While the above process is being repeated, the heat released from the surface state concentrates at the facets of the semiconductor, and this heat narrows the forbidden band width in the energy bands. Furthermore, the absorption of light increases the minority carriers, and heat generation further increases by way of the surface state. This process raises the temperature of the semiconductor surface, which may reach the melting point of the semiconductor, resulting in facet breakdown.
In the case of GaAs, an accumulation layer forms, while in the case of other materials such as AlGaAs, an inversion layer may form. In the latter case, majority carriers are trapped in the surface state, and facet breakdown occurs by the same process as that by an accumulation layer. In the case of semiconductor laser devices used under a high injection condition, heat generation originating with the surface state becomes a more serious problem.
A structure in which a window area is formed on the facet surface has been proposed as a means of preventing facet deterioration due to heat generation at the facets as described above. This method provides a transparent area with respect to the laser light on the facet surface, whereby light absorption in the facets is eliminated and heat generation caused by light absorption is suppressed. However, the process used to form the window of such a structure is extremely complicated, and the difficulty of forming an optical waveguide in the vicinity of the facets becomes a problem.
The method proposed on pages 263 to 266 in "Extended Abstracts of the 20th Conference on Solid State Devices and Material, Tokyo (1988)" attempts to improve the surface characteristics in an MIS structure using GaAs. In this method, the oxide film which has been formed on the GaAs surface in air can be removed so as to apply GaS in its place by treating the surface with an aqueous (NH.sub.4).sub.2 S solution. The formation of a GaS film makes it possible to reduce the surface state caused by the oxide film.
However, in the optical devices such as AlGaAs semiconductor laser devices in particular, improvement of the facets by the above-mentioned surface treatment has not yet been attempted. This is because aluminum is an extremely active material and its oxide film is stable, so that the removal of the oxide film is not considered possible.